This invention relates to a method and an apparatus for drying granular solids with venturi powered gas circulation, and more particularly relates to a method and an apparatus for drying plastic pellets in a continuous-flow dryer using venturi powered gas circulation of dry compressed air.
The process of drying granular solids in a continuous flow is done in a conventional manner using a funnel bottom vessel in which the material to be dried is fed into the top by gravity, and is discharged from the bottom of the vessel. During the time the material is moving through the vessel, a counter flow of a drying gas is passed into the bottom of the vessel, and disperses evenly through the granular material to remove water and other volatile substances that are present in the feed stock material. Alternatively to this counter flow method, a concurrent flow of a drying gas may be used, in which the drying gas is fed through the vessel in the same direction as the granular solids being fed through the vessel. The drying medium may be air or other selected gases depending on the desired interaction of the material and drying medium.
It is typical in the processing of granular plastic pellets to use very dry air or inert gases to remove the water or other volatiles (e.g. vapor phase substances emitted by the plastic) from the material. The flow of gas may come from several different methods of supply. One of the simplest is to use a continuous source of clean, dry gas. This is particularly desirable when the material to be dried off-gases volatile fractions that are not returned to the process, and can be removed or disposed of without concern as to its affect on the process if reintroduced. Since the drying process is continuous, a continuous flow of dry gas is required to process the material to be dried. The usual method to supply dry gas to the drying vessel is a process dehumidifier, which is typically a physically large device, or use the full required flow of dry compressed gas reduced to atmospheric pressure, and disposing of the gas after use. However, the use of xe2x80x9cnewxe2x80x9d gases in most cases will render the economics of the process unsatisfactory.
In the normal construction of these devices, the gas is moved by a fan, blower, or pump to the drying vessel to remove the water and other volatiles from the process material. The gas recovered from the top of the vessel is collected since the amount of water/moisture present is far less than the water/moisture levels found in the usual atmospheric conditions. This low dewpoint air is then further dried to reduce the water/moisture content to an absolute minimum. It can then be sent back to the drying vessel to again absorb water and other volatiles from the material being processed.
In the construction of some small devices of this type, the use of dry compressed plant air utility in a manufacturing facility is done to enable the user to work with only a minimum size vessel on the next stage of the material process, and not use the recirculating gas device due to size and space constraints.
In view of the above, there is a need for an improved method and apparatus for drying granular solids which utilizes a commonly-existing plant compressed air supply.
In addition, there is a need for an improved method and apparatus for drying granular solids which has relatively low construction costs.
Furthermore, there is a need for an improved method and apparatus for drying granular solids which has relatively low operating costs.
Moreover, there is a need for an improved method and apparatus for effectively drying granular solids which has a compact construction.
These and other needs have been met according to the present invention as discussed in the following.
The present invention advantageously utilizes a commonly-existing plant compressed air utility as a drying gas source. Rather than consume the full air flow (i.e., vent all of wet gas to ambient atmosphere) as is done by the prior art to dry the granular material, a venturi is incorporated in a recirculation loop to conserve the dry air utility, and to provide the motive power for the recirculation of the drying gas without any other mechanical devices (e.g., fan, blower, pump, etc.). Since an existing compressed air supply is used, the costs of constructing the invention are relatively low.
Another advantageous aspect achieved by the present invention is that the recirculation of a portion of the wet gas advantageously reduces operating costs (e.g., the energy consumption costs of the compressed air utility), yet still maintains a relatively dry composition of the drying gas in order to efficiently and effectively remove the water and other volatiles from the feed stock material. If necessary to achieve a desired quality of dry gas supplied to the process, the compressed air utility may be dried by a conventional desiccant drying means to a dewpoint in the xe2x88x9240xc2x0 to xe2x88x9280xc2x0 F. range. However, the compressed air itself may be dry enough without additional drying. In the process of drying plastic pellet materials in particular, the need for a drying gas in this range is not required for the proper removal of water and other volatiles from the pellets. By experience and trial data, it is possible to dry most plastic resins with air having a 0xc2x0 to xe2x88x9220xc2x0 F. dewpoint. The present invention takes this into account by venting only a portion of the moist air leaving the drying hopper to the ambient atmosphere, and recirculating the remaining non-vented portion of that moist air in a recirculation loop. The recirculation is advantageously achieved exclusively by a venturi air mover powered by the dried compressed air from an existing plant compressed air supply, the dried compressed air being added in at a flow rate to replace the proportion of the moist air vented to the ambient atmosphere to achieve a xe2x80x9cdilutedxe2x80x9d drying air stream having an overall quality very well suited to the above-mentioned process requirements, while advantageously minimizing energy requirements.
The instant invention conserves the amount of compressed gas used by providing a nominal dilution of the gas recirculating in the closed loop. The proportion of moist gas vented to the ambient atmosphere and replaced by dried compressed air depends upon the required design parameters of the particular application, including but not limited to the type of material being dried, the moisture content of the material being dried, the flow rate of the material being dried through the drying hopper, the size and configuration of the drying hopper, the dewpoint of the dried compressed air, the flow rate of drying gas supplied to the drying hopper, etc. For example, the proportion of the moist gas exiting the drying hopper which is vented to the ambient atmosphere (and replaced by dried compressed air) may be within the range of 5% to 50%. More preferably, in most applications for drying plastic resin pellets, the proportion of moist air vented and replaced by dried compressed air is within the range of 10% to 33%.
The use of the present invention for the plastic resin drying process is a remarkable improvement over the conventional dehumidifier for the smaller operations, in that the only devices required at the point of use for the plastic resin are a drying vessel, a gas heater and a venturi.
By using the compressed air utility and the venturi to power the recirculation loop exclusive of other mechanical means, the present invention advantageously achieves a relatively compact size.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.